Fast-j: Accurate simulation of in- and below-cloud photolysis in tropospheric chemical models

Photolysis rates in the troposphere are greatly affected by the presence of cloud and aerosol layers. Yet, the spatial variability of these layers along with the difficulty of multiple-scattering calculations for large particles makes their inclusion in 3-D chemical transport models computationally very expensive. This study presents a flexible and accurate photolysis scheme, Fast-J, which calculates photolysis rates in the presence of an arbitrary mix of cloud and aerosol layers. The algorithm is sufficiently fast to allow the scheme to be incorporated into 3-D global chemical transport models and have photolysis rates updated hourly. It enables tropospheric chemistry simulations to include directly the physical properties of the scattering and absorbing particles in the column, including the full, untruncated scattering phase function and the total, uncorrected optical depth. The Fast-J scheme is compared with earlier methods that have been used in 3-D models to parameterize the effects of clouds on photolysis rates. The impact of Fast-J on tropospheric ozone chemistry is demonstrated with the UCI tropospheric CTM.